The present invention generally relates to optical information professing and more particularly to an objective lens and an optical pickup for conducting at least one of information recording, information playback and erasing of information to optical media designed for blue wavelength band, red wavelength band or infrared wavelength band, as well as the information processing apparatus that uses the same.
For the means for saving video information, audio information or data in a computer, the use of optical recording media such as a CD having a recording capacity of 0.65 GB, or a DVD having a recording capacity of 4.7 GB, is spreading. In these days, there is an acute demand for further improvement of recording density for more recording capacity.
In order to increase the recording density in such an optical recoding medium, it is necessary to increase the numerical aperture (NA) value of the objective lens or reduce the wavelength of the optical source in the optical pickup used for writing or reading information to and from the optical recording medium, such that the size of the beam spot formed on the optical recording medium by the objective lens is reduced.
In the case of the optical recording medium called CD, a numerical aperture of 0.40-0.50 is used for the objective lens and a wavelength of about 785 nm is used for the optical source. In the case of the recording medium called DVD, in which the recording density is increased over the CD, a numerical aperture of 0.60-0.65 is used together with the wavelength of the optical source of about 660 nm.
As noted before, there is a keen demand for improvement of recording density and increase of recording capacity in the optical recording medium in these days, and thus, there is a desire to increase the numerical aperture of the objective lens beyond the conventional value of 0.65 and/or to decrease the wavelength of the optical source to less than 660 nm.
Thus, there appears a situation in which a new specification of higher numerical aperture value or shorter wavelength appears every year for the optical pickup, while the users of the optical information processing apparatus maintain CDs and DVDs of conventional specification. Because of this, there is arising a demand that such conventional optical recording media can be processed also in the optical information processing apparatus designed for such novel optical recording media of novel specification.
While such compatibility between different specifications can be achieved by providing a conventional optical pickup in addition to the optical pickup designed according to the new specification, such a construction is contradictory to the requirement of downsizing and cost reduction.
It is preferable to provide a construction shown in FIG. 1A in which a blue optical source 100 and a DVD optical source 200 use a single objective lens 104 that focuses the optical beams of the respective optical sources to an optical recording medium 103, such that the optical pickup, designed for a large-capacity (blue) optical recording medium by using the optical source 100 of the blue wavelength band, is still compatible with the conventional CDs or DVDs. Alternatively, it is preferable to provide a construction shown in FIG. 1B in which a blue optical source 100, a DVD optical source 200 and a CD optical source 300 cooperate with the single objective lens 2 that focuses the exit beams of the respective optical sources to the optical recording medium 1.
On the other hand, in order to focus the optical beams to the optical media designed for various different specifications such as blue color systems, DVDs or CDs, there arise various problems in relation to the difference in the wavelength/substrate thickness of the optical recording medium 103 as will be explained hereinafter.
Japanese Laid-Open Patent Publication 2002-107617 describes the problem of aberration (see FIG. 3) arising in the case an objective lens 110 of FIG. 2 designed of the optical source wavelength of 405 nm is used in the wavelength range of 400-800 nm. In FIG. 2, the reference numeral 111 shows an optical recording medium.
As will be explained below, it is generally required that the wavefront aberration should be 0.02 λrms or less. On the other hand, FIG. 3 shows that there appears an aberration of 0.10 λms or more at the wavelength of 660 nm or 785 nm used with a DVD system or CD system. In view of this situation, the foregoing Japanese Laid-Open Patent Publication 2002-107617 proposes the use of a compound lens for the objective lens. However, the use of such a compound lens raises the problems in that it requires a bonding process at the time of manufacturing of the objective lens and there arises the problem of increase of weight associated with the construction that uses two lenses.
As an alternative, there is a proposal in ISOM2001 Abstract, “BLUE/DVD/CD COMPATIBLE OPTICAL HEAD WITH THREE WAVELENGTHS AND A WAVELENGTH SELECTIVE FILTER”, or in Japanese Laid-Open Patent Application 2003-67972 to construct the respective optical paths of DVD or CD by a finite optical system in view of the fact that there occurs a spherical aberration in the event an optical beam is focused to the optical recording medium of DVD or CD by the objective lens designed for a blue optical beam. In this proposal, there is further provided a phase compensation element such that the phase distribution is changed for the optical beam of DVD or CD while not causing any change of phase distribution for the blue optical beam of the blue wavelength band. By using a finite optical system, in which the incident optical beam to the objective lens is made divergent, an effect of reducing the spherical aberration is achieved. However, there still remains some spherical aberration, and compensation of this residual spherical aberration is achieved by inducing a spherical aberration of opposite phase by the phase compensating element.
For example, in the case of using a single objective lens, which is designed for minimizing the wavefront aberration when it is used with an infinite optical system (a parallel beam is incident to the objective lens) and with a blue optical recording medium (λ1=407 nm, NA(λ1)=0.67, substrate thickness t1=0.6 mm), for focusing a beam spot on a DVD recording medium (λ2=660 nm, NA(λ2)=0.65, substrate thickness t2=0.6 mm), there arises a spherical aberration shown in FIG. 12B due to the difference in the wavelength.
Such a problem was also noticed in the DVD/CD compatible optical pickup. Thus, in the case of using a single objective lens, which is designed for minimizing the wavefront aberration when it is used with an infinite optical system (a parallel beam is incident to the objective lens) and with a DVD recording medium (λ2=660 nm, NA(λ2)=0.65, substrate thickness t2=0.6 mm), for focusing a beam spot on a CD recording medium (λ3=780 nm, NA(λ3)=0.50, substrate thickness t3=1.3 mm), there arises a similar spherical aberration shown due to the difference in the wavelength.
In order to attend to this problem, Japanese Patent 2,725,653 or Japanese Laid-Open Patent Application 10-334504 teaches the construction in which there are provided two laser diodes of different wavelengths and a phase compensating element having wavelength selectivity, such that recording or playback is conducted for a DVD optical recording medium having the thickness of 0.6 mm by using a beam of 660 nm wavelength emitted from one of the laser diodes and carries out recording or playback for a CD optical recording medium having a thickness of 1.2 mm by using the beam of 780 nm wavelength emitted from the other laser diode. Thereby, the phase compensating element having the wavelength selectivity causes no change of phase distribution for the optical beam of the 660 nm wavelength, while the phase compensating element causes a phase distribution change for the optical beam of the 780 nm wavelength in such a manner that the spherical aberration caused by the difference in the thickness of the substrate is successfully compensated for.
As an alternative, there is generally known the means of compensating for the spherical aberration caused by difference in the thickness or wavelength between the DVD optical recording medium and the CD optical recording medium, by using an infinite optical system when the optical beam of the 660 nm wavelength for DVD is incident to the objective lens and by using a finite optical system (a state in which a divergent beam enters into the objective lens in the form of a divergent beam) for the case the optical beam for CD is incident to the objective lens.
Particularly, the ISOM2001 Abstracts op. cit., proposes the method of recording or playing back information to or from any of the three optical recording media of blue optical recording medium, DVD optical recording medium and CD optical recording medium while using a single objective lens. According to the teaching of this prior art, there is provided a system including three different laser diodes of the wavelengths of 405 nm, 650 nm and 780 nm and a wavelength-selective phase compensating element having wavelength selectivity, such that optical irradiation is made to a blue optical recording medium having a thickness of 0.1 mm with the wavelength of 405 nm by using an infinite optical system, and an optical irradiation is made to a DVD optical recording medium having a thickness of 0.6 mm with the wavelength of 660 nm by using a finite optical system, and optical irradiation is made to a CD optical recording medium having a thickness of 1.2 mm with the wavelength of 780 nm by using a finite optical system. Thereby, the wavelength-selective phase plate does not change the phase distribution for the optical beam of the wavelength of 405 nm while it changes the phase distribution for the optical beams of 660 nm and 780 nm. In this construction, the wavelength-selective phase compensating means and the finite optical system used for the case of the DVD/CD recording media constitute the wavefront compensating means compensating for the spherical aberration caused by the difference in the thickness of the substrate.
In addition to the foregoing problem of aberration, there arises another problem in that the numerical aperture value changes between the optical recording media of different specifications.
Generally, the numerical aperture values NA of 0.60 and 0.65 are used respectively for the playback system and recording system of a DVD optical recording medium, while the numerical aperture values NA of 0.45 and 0.50 are used respectively for the play back system and recording system of a CD optical recording medium.
Most of the optical information processing apparatuses currently sold are capable of recording for both of DVD and CD, and it is preferable to use the numerical aperture value NA of 0.65 for DVD and the numerical aperture value NA of 0.50 for CD.
On the other hand, with regard to the blue optical recording technology, there is proposed the use of the numerical aperture value NA of 0.65 in relation to the HD-DVD technology.
This means that there is a need of switching the numerical aperture value in response to the optical recording medium used for recording and playback.
Thus, in order to achieve two-generation compatibility between DVD and CD, there is proposed a technology that uses an aperture switching element as disclosed in the Japanese Patent 3,240,846, Japanese Patent 2,713,257, Japanese Patent 2,725,653, and Japanese Patent 3,036,314. Further, in order to achieve two-generation compatibility between a large-capacity optical recording medium and DVD, there is proposed a technology that uses an aperture switching element as disclosed in the Japanese Laid-Open Patent Application 2001-216676.
Further, with regard to the technology that achieves three-generation compatibility, there is proposed the use of a three-step aperture switching element in Japanese Laid-Open Patent Application 2000-187870 and Japanese Laid-Open Patent Application 2003-67972.
With regard to such a conventional technology, the phase compensating element disclosed in the foregoing ISOM Abstract, op. cit, or Japanese Laid-Open Patent Application has to be driven integrally with the objective lens in view of the possibility of comma aberration when there is caused a displacement between the objective lens and the optical axis. With regard to the actuator that drives the objective lens in the focusing direction and the track direction, there is a demand of reducing the number of the parts in order to achieve weight reduction and reduction of the number of assembling steps. In order to suppress formation of comma aberration, there is a need for the adjustment of alignment between the objective lens and the phase compensating element.
Further, according to the method described in the foregoing ISOM Abstract, op cit., no satisfactory wave front performance is obtained when to achieve the compatibility between the DVD optical recording medium and the CD optical recording medium.
Generally, the reference value of 0.07 λ rms, known as Marshal criterion, is used as the wavefront aberration for the case of the diffraction limit. In the case of an optical pickup, on the other hand, there exist various factors that may become the cause of the error such as the thickness error of the optical recording medium, the tilt error of the optical recording medium, the defocus error caused by displacement between the optical recording medium and the objective lens. Thus, it is preferred that the wavelength aberration (median value) is suppressed to 0.03 λrms or less in the state there is caused no error, in view of the fact that there may be caused deterioration of the wavefront aberration as a result of buildup of these errors.
Contrary to this, it should be noted that the wavefront aberration (median value) becomes about 0.05 λ rms for a DVD optical recording medium in the technology described in the foregoing ISOM Abstract, op. cit. in view of the situation that it is not possible to minimize the wavefront aberration for both of DVD and CD by using a single element. Because of this, it was inevitable to make the design to target the intermediate point of phase compensation condition when to minimize the aberration in both of DVD and CD, while such an approach could not reduce the aberration sufficiently.
The same applies also to the numerical aperture switching elements. In the case of the numerical aperture switching element, it is preferable that it is driven integrally with the objective lens in view of the fact that the desired numerical aperture value is no longer attained when the optical axis of the objective lens is offset.
Generally, a numerical aperture switching element is mounted on an actuator such that the numerical aperture switching element is driven integrally with an objective lens so that the change of the numerical aperture value is avoided. On the other hand, an actuator is also required to include smaller number of parts for weight reduction and reduction of the number of the assembling steps. Thus, it is desired that such a numerical aperture switching element is removed from the actuator. Particularly, the numerical aperture switching element designed for three-generation optical recording and playback so as to operate in three steps has a complex structure, and design and manufacture thereof becomes difficult. Further, in spit of its complex construction, such a three-step numerical aperture switching element cannot provide sufficient compensation with regard to the wavefront characteristics and transmission characteristics.